1. Field of the Invention
This invention relates to interface electronics incorporated into card readers, and in particular interface assemblies enabling communication between a plurality of respective card readers and a central processing unit.
2. Description of the Prior Art
Card reader units for reading paper cards containing punched or marked information have been in widespread use in the electronic data processing area for some years. The versatility of card readers as input elements in larger information processing systems is greatly enhanced by the ability of interfacing the reader directly with a central processing unit (CPU). Such interfacing requires interface electronic circuitry for generating and transmitting commands from the card reader to the CPU, as well as for receiving and coding commands coming from the CPU to the card reader. Additionally, and perhaps most fundamentally, an interface is required to convert the normal card reader TTL level parallel output lines to an RS232 level serial bit stream suitable for use with a CPU or other data communications equipment. It is desirable that the interface package be wholly contained within the card reader electronics. It is further required of the interface that it convert card reader information from Hollerith to ASCII, provide for retransmission of a previous card's data as often as requested, and be compatible with punch card readers as well as optical mark sense card readers, in both the 40 column and 80 column mode.
In addition to the above, there is a great need for a card reader interface assembly which provides for a flexible easily incorporated means for generating and transmitting status characters from the card reader to the central processing unit, so that the card reader may communicate quickly and efficiently with the CPU. Likewise, there is a great need in the industry for card reader interface electronics which cooperates with the CPU so that a plurality of card reader units may be programmably interfaced with the CPU, either singly or in combination.